A radiation oncology treatment system must be properly calibrated to ensure that the radiation beams accurately target the treatment volume. Calibration of the radiation isocenter is one of the quality control procedures to determine the accuracy of the treatment system. In short, the radiation isocenter is the point in space where radiation beams intersect when the gantry rotates around the gantry rotation axis. More specifically, the point where the central radiation beams intersect is a small volume that looks geometrically similar to a sphere or an ellipsoid, and the center-of-mass of this volume is the radiation isocenter. The radiation isocenter differs from the mechanical isocenter; the radiation isocenter is the point in space through which the central beam of radiation passes, whereas mechanical isocenter is the point where the targeting optical beams intersect (or where the gantry rotation axis and the collimator rotation axis intersect). When properly calibrated, the targeting optical beams of the treatment system that are used to target the radiation beams at intended target should intersect at radiation isocenter, but the two isocenters do not necessarily coincide.
Traditionally, the radiation isocenter is found by exposing radiographic film with a star shot pattern. The film is placed so that the beam enters through the edge of the film when it is irradiated from several gantry angles with a small field. The rotation axis of the gantry, running perpendicularly through the film, is marked. The middle lines of the three radiation beams, shown as stripes on the film, form a triangle when they intersect in the film center. The smallest circle which fits inside the triangle (all three beams have to intersect the circle or at least touch it) is usually called the radiation isocenter circle. Its size (diameter or radius) can be used to determine a quality parameter of the linac.
An alternative to find the radiation isocenter is the Winston-Lutz test (WL test), which characterizes specific aspects of Clinac treatment beam dose distribution error based on treatment beam test images. The Winston-Lutz test is becoming more widely adopted as the test for geometric positioning accuracy of the Clinac for its simplicity and similarity to the actual treatment: the cone represents the beam-shaping elements (e.g., the collimator), and the ball represents the tumor, located nominally at isocenter. The WL test if performed by having a lead ball on a rod is held in place and aligning the center of the lead ball with the targeting optical beams. The lead ball is then exposed to film or digital imager in different combinations of the gantry angles and the table angles. The isocenter is then calculated by locating the center point of the image of the ball, which is done by using conventional method of finding the mid point of the ball image via a ruler.
The Winston-Lutz test has many drawbacks, however. The beam scattering and penumbra effects will cause an unavoidable blurring of the image edge thus resulting in uncertainty of the true edge of the ball image and the calculated eccentricity. The resolution of the film and digital imager must also be high (˜0.1 mm) in order to accurately find the edge of the ball image under magnification. In addition, visually estimating the image edge is susceptible to random noise present in the image, especially in conventional films.